Service Entrance Voltage Drop Planning
Service entrance work looks simple when people only watch the service rating. It is not. A 200A or 320A service can meet ampacity rules and still waste too much voltage if the run from the service point to the main equipment is long or the downstream distribution already uses most of the design margin.
The practical review is to combine load calculation, conductor material, one-way route length, and the voltage-drop budget for the whole path. For NEC work, useful checkpoints are Articles 220, 230, 310.12, and the familiar informational notes in 215.2(A)(1) and 210.19(A)(1). For IEC-style work, IEC 60364-5-52 remains the working reference. NEC IEC
Why Service Entrance Runs Deserve Their Own Voltage-Drop Check
The service or main feeder often crosses long driveways, large homes, shops, barns, or detached structures before branch circuits even begin.
Dwelling conductor sizing under NEC 310.12 may satisfy ampacity, but long runs can still justify the next conductor size for better voltage at the panel.
Aluminum service conductors are common and economical, yet their higher resistance means distance matters sooner than many DIYers expect.
A weak service entrance uses up the same 5% design budget that downstream feeders and branch circuits also need.
Code and Standards Notes to Keep on the Worksheet
- NEC Article 220: start with a believable load calculation before discussing conductor size or service rating.
- NEC 230.42: service entrance conductors still need adequate ampacity and installation conditions for the calculated load.
- NEC 310.12: dwelling services and feeders may use simplified sizing, but voltage-drop planning can still push the design larger.
- NEC 215.2(A)(1), 210.19(A)(1), and IEC 60364-5-52: many designers still target about 2% at the service or feeder when they want to preserve room for downstream circuits inside a 5% total plan.
Quick Planning Numbers for Common Service Entrance Scenarios
These are planning examples based on standard conductor resistance values. Final design still needs the actual route, ambient conditions, termination ratings, and local utility or AHJ requirements.
| Scenario | Distance and load | Conductor | Approx. drop |
|---|---|---|---|
| Dwelling service | 180 ft one-way, 200A, 120/240V | 4/0 AWG Al | about 5.8V / 2.4% |
| Dwelling service | 180 ft one-way, 200A, 120/240V | 250 kcmil Al | about 4.9V / 2.0% |
| Shop or farm service | 220 ft one-way, 320A, 120/240V | 600 kcmil Al | about 4.0V / 1.7% |
| Small building feeder | 45 m one-way, 100A, 230V | 35 mm2 Cu | about 4.4V / 1.9% |
Worked Service Entrance Examples with Specific Numbers
200A dwelling service, 180 ft one-way
Using 4/0 aluminum at about 107 mm2 gives roughly 5.8V drop, or 2.4% on a 240V service. That still works in many homes, but if the house also has long EV, HVAC, or workshop branch circuits, 250 kcmil aluminum at about 2.0% is usually the cleaner planning choice.
320A residence, barn, or shop service, 220 ft one-way
At this distance, 400 kcmil aluminum lands near 2.5% while 600 kcmil aluminum drops to about 1.7%. The larger conductor can protect motor starts, large heat-pump loads, and future branch-circuit margin better than the minimum ampacity answer.
230V single-phase outbuilding, 100A, 45 m route
A 25 mm2 copper feeder is near 6.2V or 2.7%. Moving to 35 mm2 cuts it to about 4.4V or 1.9%, which is often easier to live with when the final circuits inside the building still need their own drop allowance under IEC practice.
Field Checklist Before You Lock the Service Conductor Size
- Measure the real one-way route from service point or meter position to the actual main disconnect or distribution equipment.
- Separate service conductor sizing, feeder sizing, and branch-circuit sizing so one weak segment does not consume the whole design budget.
- Compare copper and aluminum using the same route and load instead of choosing by habit alone.
- Review future EV charging, HVAC, welders, or detached-building loads before finalizing the service conductor size.
- Keep utility requirements, NEC or IEC rules, lug ratings, and voltage-drop math on the same worksheet.
Run the Service Entrance Numbers Before You Pull Big Conductors
Use the service entrance calculator to compare conductor materials, route lengths, and practical service-drop targets before the meter base, trench, or main panel location is fixed.
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